The invention relates to a dental prosthesis blank, whose end, which is not to be cut, is connected to an essentially cylindrical cone, and a blank holder with radially moveable gippers for the dental prosthesis blank.
A dental prosthesis blank, as disclosed, for example, in the EP A 759 728, is made of a precious metal, a titanium alloy, or a sintered zirconium oxide or the like. Usually it exhibits a cylindrical body, to which a cone is molded that is also cylindrical and that usually tapers off radially. To be able to cut the individual dental prosthesis from the body, the blank must be clamped so as not to rotate in a blank holder during the machining process. As soon as the machining is completed, the blank is removed from the holder, whereby the dental prosthesis is still attached to a body stump.
It can happen that, after the dental prosthesis has been accurately measured, it is necessary to rework it; and, therefore, the blank must be clamped again into the holder. The subsequent work is usually done by hand, a state that requires not only considerable skill, but also does not often lead to the desired result
Therefore, the invention is based on the problem of simplifying the reworking of a dental prosthesis connected to the body trunk of a blank.
To this end, the invention provides for the aforementioned blank that the cone exhibits at least one reference element, which interacts with a spatially fixed reference counter element so as to position accurately the blank in the circumferential direction when clamping the cone into a blank holder. The reference element and the reference counter element form a spatial reference system, which always spatially fixes the blank, inserted into the blank holder, in a precisely specified circumferential direction around the blank axis so that, when the dental prosthesis is being machined out of the blank body, the blank can be inserted arbitrarily often into the holder and removed again from the same without thus changing the spatial position of the dental prosthesis. Therefore, it is possible to easily rework the dental prosthesis with the necessary accuracy.
In the preferred embodiment of the invention the reference element is a flat reference surface, which extends expediently parallel to the axis of the cone. An especially preferred embodiment of the invention provides that the cone is slit axially; and the reference element is formed on or in the slit. Here it is especially recommended that one flat reference surface be formed at each of the two side walls that belong to the slit and lie opposite the axis of the cone. The reference counter element can be a free leg or a rib, which is formed in the blank holder. Said reference counter element extends at right angle to the cone""s direction of insertion into the holder and has a radial width, which is equivalent to precisely the distance between the two reference surfaces on the slit cheeks. Thus, the slit floor, connecting the two reference surfaces, can serve as the axial reference, which also interacts with the leg or the rib when clamping the blank into the holder.
The accuracy of the blank alignment in the holder in the circumferential direction is increased in another improvement of the invention when the cone is slotted crosswise, whereby the respective cheeks of the cross slots are designed as flat reference surfaces, parallel to the axis.
Moreover, the embodiments of the invention are disclosed in the dependent claims. The invention is described in detail in the following with reference to two embodiments depicted in the attached drawings.